and also of the relative quantity of black and white exposed in the black and white discs; then another trial is made in the same way, only that the black and white discs are altered so that a different proportion of the two colours are exposed. It will now be found that equal luminosity of the colour patch on the discs occurs when the slit is in a different part of the spectrum to what it was before the two measurements were made. These measurements are repeated for various proportional exposures of the black and white discs. Then on a chart is drawn two sets of lines, viz., a horizontal set, to show the proportions of black and white; and a vertical set, to show the position of the slit on the spectrum. Then drawing a line through the points given by the various readings, a curve is obtained indicating the reflecting power of the particular pigment experimented with in each part of the spectrum.

By carrying out this system with different pigments we are able to see how one compares with another.

By a modification of the experiment the light reflected from a surface painted with a pigment can be compared with that which is reflected from a surface painted with a standard sample of the pigment. If instead of laying down the curve on a chart, it is drawn on a sector of a circle, so that the scale of the spectrum is measured off along a radius and the relative intensities on concentric circles, then a curve of somewhat different shape is obtained. If this be cut out it forms a colour template which, when revolved in front of the spectrum formed in the apparatus, cuts off just enough light that the remainder forms a colour patch of the same hue as the colour pigment; by causing a patch of colour to be formed side by side with the colour patch, the relative hues may be accurately compared together. This second patch of colour is obtained by reflecting from a surface painted with the pigment. By making templates in this way and using the colour patch apparatus in the manner indicated, there is a certain method of comparing hues of pigments.

THE TINTOMETER.-Mr. J. W. Lovibond of Salisbury has devised, for the purpose of making colour measurements, an instrument which he has named "the Tintometer." This instrument consists of a rectangular pyramidal tube with an eyepiece at the smaller end. The tube is divided by a longitudinal division so that it is possible to view two different coloured objects at one time and compare their colour or tints together. To apply the instrument for colour measurement, there is provided a set of standard coloured glasses which can be placed in suitable grooves in the tube of the instrument. To apply the tintometer to the

examination of pigments, the following plan may be adopted. The instrument is placed in an inclined position in front of a well-lighted window. A small shallow box filled with the colour to be examined is placed under one half of the tintometer and a similar box filled with precipitated sulphate of lime under the other half, the surfaces of both bodies being flattened. There is then obtained reflected coloured light in one half of the instrument and reflected white light in the other. In this half of the tube is now placed one or more of the standard colour glasses until the colour and tint of the pigment is exactly matched; the number of glasses used is then recorded for future reference.

BRILLIANCY or LUMINOSITY.-This is an important feature of pigments, and one in which different makes of the same pigments are rather liable to vary. As with hue, brilliancy is assayed by comparison with a standard sample, and this can be done in precisely the same manner as described for hue.

COLOURING POWER.-Colouring power is that property of pigments which enables them to give colour to surfaces and to other pigments. As explained in another place, pigments possess three properties available for paint making, viz., colour, body, and covering power (see below). Some pigments are used almost solely on account of their colour, as, for instance, carmine, Prussian blue, ultramarine, vermilionettes; hence with these strength of colour or colouring power is an important feature. Other pigments are used solely on account of their covering power, and then colour is immaterial.

Colouring power is tested also by comparison with a standard sample. In principle it is done by ascertaining how much of another pigment it will colour to a given depth.

Supposing it is a sample of vermilionette whose colouring power is to be determined, then 10 grms. of the sample are weighed out and mixed with 30 grms. of china clay; the mixing must be thoroughly done. 10 grms. of the standard sample are mixed in the same way with 30 grms. of the same sample of china clay. The two mixtures are now compared together for depth of colour as described above; if the two samples are equal in colouring power, the depth of colour of the two mixtures will be the same; if one is stronger than the other, then one of the mixtures will be darker than the other. Some idea of the relative strength of colouring power may be obtained by adding small and known weights of china clay to the darkest sample until the tint of the mixtures are equal to one another; then the samples have a colouring power proportional to the amount of china clay used; thus, if one sample took 30 grms. of china

clay and the other sample 375 grms., then the relative colouring power is as 30 to 375; or, if the strongest sample be taken at 100, then the colouring power may be expressed in percentages thus, 37.5 30: 100: 80; the weakest colour has only 80 per cent. of the colouring power of the strongest.

Again, in making some experiments to test the comparative colouring powers of Orr's white and white lead, 5 grms. of the former were mixed with 1.46 grms. of blue, and the tint thus formed was found to be exactly matched by a mixture of 5 grms. of white lead with 0.55 grm. of blue. Hence we have

14655: 100: = 236'6

that is, 100 parts of Orr's white is equal to 236-6 parts of white lead as regards colouring power.

A series of experiments carried out to ascertain the colouring power of various white pigments resulted in the following figures being obtained. 10 grms. of each white was taken and mixed with a quantity of black to produce a uniform grey tint, the amount of black so used was noted and was as follows:

As the toning colour for all pigments except whites a good sample of china clay may be used; gypsum also makes a good toning colour; barytes and white lead are a little too heavy. For whites a good animal black makes a good toning colour.

When a large number of assays for colouring power have to be made a standard tint should be made by taking, say, 50 grammes of the standard sample, and mixing with about twice its weight of the toning colour; this tint may be used in subsequent tests, and will save some time in the preparation of a standard tint. It is important, however, that the same sample of toning colour be used to mix with the samples, whose colouring power is being tested, as has been used in making the standard tint.

COVERING POWER, BODY.-Covering power and body are terms used by colourmen and painters as being more or less

synonymous, but it is quite possible, nay, indeed, it is almost certain, that under these terms two different properties of pigments have been and are confused together.

By covering power is really meant the spreading power of a pigment-that is, when mixed with oil, &c., into a paint of suitable consistency, this paint can be spread over a certain extent of surface. Now, if equal weights of various pigments be taken and mixed with oil (sufficient being used in each case to bring them into a proper consistency for laying on) and each paint be spread over a surface as far as it will go, care being taken to get as even a coat as possible, a comparison can be made; that which covers the most surface has the greatest covering power or spreading power (see below).

By body is meant the power of hiding the surface over which the pigment or paint is spread. Thus, while the covering power of zinc white is better than that of white lead, its body is far inferior.

It will be well to consider these two properties together, for it is rather difficult to differentiate between them. Some pigments, such as crimson lake, Prussian blue, and barytes, are deficient in body although good in covering power; others, such as white lead and chrome yellow, have excellent body, but often do not cover well; while others, such as Orr's white and ochres, possess both good covering powers and good body. In the same pigment both the covering power and the body are liable to vary to a greater or less extent. To some extent the covering power and body are dependent upon the condition of the pigment: if this is of an amorphous character, without any definite form of its own, and is opaque, it will, as a rule, be found to have good covering power and body; on the other hand, if a pigment is of a transparent character, and is crystalline in its structure, then its body is liable to be small, although it may cover well. Sometimes a pigment may be obtained in both conditions, according to the particular circumstances under which it is made; thus, lead chloride may be obtained as a white amorphous powder, or in small transparent crystals. In the former condition it may be used as a pigment, as it has some body; on the other hand, the crystalline variety is useless as a pigment, as it has no body at all.

Unfortunately it is by no means an easy matter to devise a method of assaying the covering power and body of pigments; they cannot be expressed in absolute figures as can chemical composition, at the most they can only be assayed in a comparative manner as is colouring power.

The best plan for assaying the covering power and body in pigments is to place 2 grammes of the standard sample and of the pigment to be compared with it on a block porcelain tile, and to add oil to each sufficient to allow them to flow; the oil and pigment are thoroughly incorporated by means of a palette knife, and then each is spread over the plate in a layer, making each layer of paint of as uniform a thickness as possible. That sample which, when thus made into a paint and spread over the tile, covers the most surface has the most covering power.

The following may be taken as an example of the method :— 2 grammes of samples of the following white pigments were weighed out and mixed sufficient raw linseed oil as would enable them to be rubbed ver the surfaces of glass plates, then the extent of surface covered by each was observed, the result being as follows

From these it is seen that Orr's white has the greatest covering power, and is followed, in order, by gypsum, zinc white, barytes, the White Lead Co.'s product, Gardner's white lead, white lead, and lead sulphate.

The quality of a pigment must not be judged solely from the point of view of its covering or spreading power, as gypsum would then be considered one of the best of pigments for oil, while, as a matter of fact, it is one of the worst; there must be taken into consideration also the colour or staining power of the pigment and its body. The colour or staining power of the pigments in question is judged by how far they retain their whiteness on being mixed with oil; in this respect the samples named above ranged themselves in the following order Gardner's white lead, Orr's white, zinc white, lead sulphate, white lead, White Lead Co.'s white, barytes, and gypsum; the latter two were rather deficient in this respect.

By body is meant the opacity of the coat of paint which is obtained in carrying out the process described above. If the painting has been done on a black ground, and one appears to obscure the black more than another, that one will have the most body. If the paint has been spread on glass, that which